edit: just compared the 8gb 30mbps to the 30mbps extreme 3 4gb card in a deeply unscientific test...
2* D300's next to each other, hitting motor drive at 8fps on both, shooting jpeg holding the two shutters for about 3 seconds and then letting go and watching to see which LED went out first.
The 4gb card went out first by quite a decent amount of time, so yeah, I'm still happy with the 8gb card, it'll be fine as a 3rd card for each camera, but it does make me want the 8gb 60mbps cards that much more...